1. Field of the Invention
The present invention relates to heating elements including an improved means for electrically interconnecting a buss bar and an electrode.
2. Description of the Prior Art
Expandable and/or contractable heating elements including polymers which are made electrically conductive by dispersing therein suitable amounts of finely divided conductive fillers are well known in the art, and such heating elements are described in U.S. Pat. Nos. 4,177,446; 4,223,209; and 4,318,220; all of which are owned by the assignee of the present invention. The disclosures of each of these patents is hereby incorporated herein by reference.
Such heating elements are particularly useful as components of articles which comprise a heat-responsive sheet material which experiences dimensional changes upon being heated above a predetermined temperature. The heating element may be in direct contact with the sheet material, for example, secured thereto by an adhesive, or may be separated therefrom by an intermediate layer provided that there is adequate heat transfer between the heating element and the sheet material. The article is preferably flexible, at least at the temperature at which the sheet material becomes responsive.
The term "heat-responsive" is used herein to mean that when the sheet material is heated to a suitable temperature, it (a) undergoes a spontaneous change in at least one dimension in a plane thereof; and/or (b) undergoes some other change, e.g., it softens (including flows), which substantially reduces the external forces, (e.g. manual forces) required to change at least one dimension of the sheet material in the plane thereof. The sheet material preferably comprises an organic polymer, for example, a polymeric film which is heat-recoverable or can be rendered heat-recoverable, an adhesive (for example, a hot-melt or heat-activatable adhesive) or a mastic.
The heating element in such articles is placed adjacent the sheet material in such a way that the sheet material is capable of changing its dimensions in the required way when the article is heated. Such articles generally have one heater element and one sheet material, but may contain more than one element and/or more than one sheet material; for example, they may comprise an element sandwiched between two sheet materials or one sheet material sandwiched between two elements.
When the sheet material is a polymeric film which is heat-recoverable or can be rendered heat-recoverable, it preferably comprises a crystalline cross-linked polymer. Suitable polymers for heat-recoverable sheet materials are well-known in the art, see for example, U.S. Pat. No. 3,086,242, the disclosure of which is hereby incorporated by reference, and include polymers of one or more olefins and/or one or more ethylenically unsaturated monomers containing polar groups.
Heating elements of the type described are inherently weak mechanically, and accordingly, a need exists to strengthen the heating elements to accommodate stresses encountered when the sheet material undergoes dimensional changes. Although a coating having a mechanical strength could be applied to the heating element, and especially electrodes thereof, to provide the desired strength, a problem then exists in connecting buss bars with the electrodes electrically, since the coating acts as an insulator.